Corrosion-resistant surface



Jan. 23, 1962 INVENTOR.

LUTHER E. VAALER BY M mm 19 ATTORNEY United 3,017,696 Patented Jan. 23, 1962 ice 3,017,696 CORROSION-RESISTANT SURFACE Luther E. Vaaler, Columbus, Ohio, assignor to The Griscom=Russell Company, Massilon, Ohio, a corporation I of Delaware Filed Mar. 11, 1957, Ser. No. 645,287 2 Claims. (Cl. 2--527) This invention relates to the production of corrosionresistant surfaces for metals and, more particularly, to a surface for iron or steel that will effectively avoid corrosion over an extended period of time.

The problems of corrosion and/or oxidation of metals, and particularly iron and steel, have been in existence from the day metals were first refined. There has been a constant search for more eliective means to reduce or eliminate corrosion. In the case of iron and steel, efiective methods for reducing corrosion have been made only on a temporary basis. In other words, regardless of the commercial. coatings applied today, if the product is exposed to ordinary outdoor atmospheric conditions, corrosion eventually takes place and it is usually within a period of a few years or less. Iron and steel have been galvanized, sherardized, hot-tin dipped, electrolytically plated, chemically coated with phosphates and black iron oxides, and greased or oil-coated. None of these coatings ofiier resistance to corrosion in ordinary atmosphere beyonda few years, and very few of these coatings provide an attractive bright surface desirable for many applications. Alloying has proved to be too expensive to use in most cases for corrosion resistance alone and ordinary alloying of metals, such as iron or steel, will not otter protection over an extended period of time.

For many outdoor applications, it is desirable to employ a bright and lustrous metal finish. Such an application is automobile trim and the decorative trim used on building fronts, etc. The most commonly used coating for such application is chromium electroplate. It is well known that under ordinary atmospheric conditions chromium-plated building trim will corrode within a few years. Chromium-plated auto parts, due to somewhat more severe corrosive conditions, may show signs of corrosion within a few months, unless considerable care is taken to keep the parts free of corrosive materials.

It has now been found that a bright and lustrous surface may be produced that will offer substantially increased resistance to corrosion over an extended period of-time.

In patent application Serial No. 585,088, there is disclosed a heat-transfer device wherein a porous metal surface is impregnated with dropwise promotion materials. One of the surfaces described is a chromium electroplated surface which contains capillary openings in the chromium plate and reservoirs in the metal lining below. The reservoirs are impregnated with a dropwise promoting and corrosion-inhibiting material which is continuous with a dripwise promoting and corrosion resistant film covering the chromium plate. The present application is a continuation-in-part of patent application Serial No. 585,088.

It is the object of the present invention to provide a corrosion-resistant surface.

It is also an object of the present invention to provide a chromium-plated surface that will exhibit superior corrosion resistance over an extended period of time.

A further object is to provide a bright chromiumplated steel object that will exhibit superior corrosion resistance to prior art chromium-plated objects.

Other objects and advantageous features will be obvious in the following specification.

In general, the present invention relates to an electroplated metal article provided with a film of a corrosionresistant material and formed with a. plurality of reservoirs and pores with opening-s to the surface of the arti cle. The pores or reservoirs and openings are impregnated with a corrosion-resistant material capable of migrating to the surface by capillary attraction replacing 'lo-st portions of the corrosion-resistant film. The invention relates particularly to chromium-plated iron or steel articles with pores opening to the surface and impregnated with a corrosion-resistant material which is continuous with a film of such material on the surface of the chromium electroplate. The corrosion-resistant material must be capable of capillary migration.

In the drawings, the figure is a fragmentary sectiona view, highly magnified, of a steel article provided with an impregnated porous metal coating.

Referring to the drawing, a base metal 10 is provided with an electrodeposited metal plate 11 over which there is provided a thin film of a corrosion-resistant material 12, such as a wax or oil, etc. The plate 11 was electrodeposited in such a manner as to provide microscopic cracks or capillary openings in its surface which, in turn, were widened by acid etching, as at 13. Reservoirs 14 were provided in the base metal by acid etching through the cracks or capillary openings 13 of plate 11. Reservoirs l4 and capillary openings 13 are impregnated with the corrosion-resistant material of film 12 which is continuous with the film. I

When exposed to an oxidizing or normally corrosive atmosphere, the wax or oil film 12 provides added corrosion resistance to the already corrosion-resistant plate 11. When film 12 is eroded by weather conditions, such as rain and wind, the film is automatically replenished by the corrosion-resistant material present in reservoirs 1d and capillary openings 13. This is caused by capillary attraction and migration of the material 11 and occurs naturally.

In accordance with the present invention the oorrosio resistance of any metal coating, and particularly electroplated metals, may be greatly enhanced by providing pores or reservoirs beneath the surface and by providing the surface of the coating with a film of a corrosionresistant substance, capable of capillary migration, which is also impregnated into the pores themselves. The reservoirs need not be located in the base metal, as illustrated by the drawing, but may be located entirely in the coating or plate itself. Obviously, more than one coating or electroplate may be provided so long as there are pores opening to the surface of the topmost plate. Thus, a metal article may be provided with a plurality of electroplates with the reservoirs being located in either the base metal or any of the plates themselves, but opening to the surface of the topmost plate.

An ideal structure is a steel article on which there has been electrodeposited a copper plate and a chromium plate has been electrodeposited on top of a copper plate. The electrodeposited chromium plate contains numerous microscopic cracks and fissures which upon electrolytic etching in a corrosive medium, such as an aqueous solution of CrO are broadened to serve as capillary openings. Further etching in nitric acid serves to provide reservoirs in the underlying copper plate. The article is now immersed in a bath of a corrosion-resistant material, such as oleic acid, and a vacuum is provided to the tank or container holding the bath. 1 When the vacuum is released, the cracks or fissures, which serve as capillary openings, and the underlying reservoirs are substantially filled with the oleic acid. The surface may now be cleaned, allowing a thin film of oleic acid to remain on the chromium plate.

The materials with which the porous surface may be impregnated may be any substance capable of forming a corrosion-resistant film and that possesses suflicient viscosity to permit capillary attraction or migration.

Ideal substances are organic materials that contain in their molecular structures polar groups and nonpolar groups. The fixed oils, i.e., those containing oleic, lauric, or palmitic acid, etc., obtained from vegetable or animal sources, differ from the pure mineral oils in their molecular structure. The fatty acids usually have a hydroxyl- OH or carboxyl-COOH radical at one end. Some com pounds which had been synthesized and contain a halogenCl or Br or an amine NE -behave in a similar manner. It is possible to replace the hydroxyl of the fatty acids by metal (sodium or potassium usually) to form a metallic soap. The fixed oils often contain unsaturated molecules in which the region of the double carbon bonds are chemically active and will readily pick up oxygen at higher temperatures. These materials form monomolecular films on the surface of metals, the polar groups contribute to the adherence of the individual molecule, and thus the film as a whole, and the nonpolar groups serve to repel water. Examples are the fatty acids such as, oleic, lauric, palmitic, stearic, and linoleic .acids, and the organic amines such as dodecylamine and octadecylamine. These compounds are referred to in the present application as polar-nonpolar materials. Ideal polar-nonpolar materials are disclosed in US. Patents 2,460,259 and 1,995,361. Also, an article disclosed in Engineering (a British journal) entitled Drop-Wise Condensation on a Metal Surface by H. Hampson, April 15, 1955, p. 467, reveals many of the desirable polar-nonpolar materials. Additional materials that may be employed are silicone compounds, Waxes, compounds containing waxes, various synthetic resin, paraflins, beeswax, etc.

A severe test of the corrosion resistance of a surface is illustrated in heat-exchanger devices wherein the surface is subjected to the severe erosion and corrosion effects of condensing water vapors. It is known that in such a device a film of a polar-nonpolar organic compound one molecule thick on a nonporous metal surface promotes the dropwise condensation of steam for about 50 hours at the condensation rate commonly used in practical applications. The surface area occupied by a single oleic acid molecule oriented with its polar group towards the surface is about 2.58 1O- square inches. Such a monolayer represents about 1.89 grams or a volume of 1.29 10 cubic inches per square inch of surface. By providing reservoirs, such as are illustrated in the drawing, the average amount of oleic acid available to the surface may be increased to 6.64 1O grams or 4.52 x 10" cubic inches per square inch of surface, which would resist erosion and corrosion, and promote dropwise condensation in the condenser for a period of years. By providing approximately 3.31 10- grams of oleic acid or 2.26 10- cubic inches per square inch of surface the resistance would be extended from 50 hours to a full year. Obviously, in ordinary atmospheric conditions, such a film of material would last far longer than when subjected to the severe corrosion and erosion conditions of a condenser surface. These figures are essentially the same for all of the polar-nonpolar molecules, in that the organic radical at one end of the molecule which has a strong affinity for the metal surface will take up essentially the same area regardless of what polarnonpolar compound is employed. The polar-nonpolar compounds that are useful as an impregnated corrosionresistant material form films of similar thickness. Therefore, the above figures given for oleic acid are approxi- J mately the same for all the useful polar-nonpolar compounds.

As an example of the present corrosion-resistant surface, chromium was electrodeposited on /1 inch O.D. copper tubes from a bath containing 239.0 grams per liter of CrO and 1.8 grams per liter of sulfuric acid. Electrodeposition was continuous for 60 minutes at a current density of 4-25 amperes per square foot with the bath maintained at a temperature of 140 F.i2 F. Electrolytic etching of the deposit was carried out in a bath containing 240 grams per liter of CrO Etching was carried out for 23 minutes at a current density of 360 amperes per square foot with the bath held at 122 F.i2 F. Further etching of the copper basis metal was carried out chemically by immersing the tube in a 50 percent solution of nitric acid for 10 minutes at a temperature of F.:5 F. The rinsed and dried tube was vacuum impregnated with oleic acid. Three-foot tubes treated in this manner were mounted in a steam chamber and cooled internally with running Water. After hours of operation, dropwise condensation and a high rate of heat transfer were still maintained, thus showing the resistance of the monomolecular film to erosion. Shorter sections of tubing mounted in a steam chamber at a steam pressure of 3 inches of water and cooled internally with a water velocity of 350 cubic centimeters per minute continued to maintain dropwise condensation on that surface even after 500 hours. After testing there was no corrosion on the surface of any of the samples.

The above example illustrates one embodiment of the present invention and in no way limits the invention to the exact embodiment set forth.

What is claimed is:

1. The method of forming a corrosion-resistant article which consists in depositing a protective metal coating upon a surface of a metal base in such a manner as to provide a plurality of pores in said protective metal coating, widening said pores and forming reservoirs at the interface between the metal base and the protective metal coating by acid etching, and coating said protective metal coating and impregnating said pores and reservoirs with a viscous noncorrosive, corrosion-resistant material capable of migrating from the pores to the surface of the protective metal coating.

2. The method of forming a corrosion-resistant article which consists in depositing a protective metal electroplate upon a surface of a metal base in such a manner as to provide a plurality of pores in said protective metal electroplate, Widening said pores and forming reservoirs at the interface between the metal base and the protective metal electroplate by acid etching, and coating said protective metal electroplate and impregnating said pores and reservoirs with a viscous, noncorrosive, corrosion-resistant material capable of migrating from the pores to the surface of the protective metal electroplate.

References Cited in the file of this patent UNITED STATES PATENTS 2,187,086 Koehring Jan. 16, 1940 2,196,261 Howland Apr. 9, 1940 2,598,213 Blair May 27, 1952 2,656,595 Stern Oct. 27, 1953 2,663,928 Wheller Dec. 29, 1953 2,689,380 Tait Sept. 21, 1954 2,793,571 Way May 28, 1957 

1. THE METHOD OF FORMING A CORROSION-RESISTANT ARTICLE WHICH CONSISTS IN DEPOSITING A PROTECTIVE METAL COATING UPON A SURFACE OF A METAL BASE IN SUCH A MANNER AS TO PROVIDE A PLURALITY OF PORES IN SAID PROTECTIVE METAL COATING, WIDENING SAID PORES AND FORMING RESERVOIRS AT THE INTERRFACE BETWEEN THE METAL BASE AND THE PROTECTIVE METAL COATING BY ACID ETCHING, AND COATING SAID PROTECTIVE METAL COATING AND IMPREGNATING SAID PORES AND RESERVOIRS WITH A VISCOUS NONCORROSIVE, CORROSION-RESISTANT MATERIAL CAPABLE OF MIGRATING FROM THE PORES TO THE SURFACE OF THE PROTECTIVE METAL COATING. 